Advances in Science and Technology Vol. 77

Abstract: In present work a new theoretical approach based on the modified three-element Eyring-Halsey mechanical model was used for the derivation of an equation, which describes the thermally-induced recovery of preloaded covalently crosslinked polymer. This approach takes into account the influence of crystallizable polymer network as well as of entangled slipped molecular chains. Modeling of the temperature dependences of shape-memory (SM) recovery strain and SM recovery rate detected at constant heating rate has been performed for three types of polyethylene with sufficiently different crystallinity and crosslink density at programming strain of 100%. The results of modeling agree well with the experimental data. The values of material parameters determined by fitting correspond satisfactorily to the estimations existing in literature. It is shown that the contribution of the entangled slipped molecules to the total stored SM strain increases with increasing degree of branching and crosslink density. The physical sense of main fitting parameters and their dependences on the material constants such as crystallinity are discussed.

Abstract: In-situ photopolymerization of liquid crystalline (LC) monomers has proven to be a valuable technique for the formation of well-ordered polymer networks. Their anisotropic properties led to a variety of applications in optics, electronics and mechanics. The use of light to initiate polymerization enables lithographic patterning. In addition the LC behavior enables formation of complex morphologies on molecular level. Controlling the director profile of an LC network film in transversal direction gives geometrical morphing upon minor changes in order parameter. Examples of suited profiles of molecular orientation are twisted or splayed director configurations tied up in the network configuration. Reversible order parameter changes can be induced by light using the photo-activated trans-cis isomerization of a copolymerized azobenzene monomer. This is demonstrated in photoresponsive cilia inkjet printed on a substrate. The cilia possess a splayed molecular organization and show well-controlled bending when addressed by light. We demonstrate a patterned film with alternating helicoidal and perpendicular-uniaxial molecular orientation. When applied as coating on glass, photo-activation in this case leads to a dynamically switching surface topology

Abstract: pH-Responsive polymers have attracted much attention for biotechnology applications as carriers or matrix to facilitate intracellular or extracellular therapeutic drug delivery and release. In this paper, we report the development of new pH-responsive and hyperbranched copolymers with potential for such applications. These pH-responsive hyperbranched copolymers were synthesized via one pot reversible addition-fragmentation chain transfer (RAFT) copolymerization of propylacrylic acid (PAA) and a branching co-monomer poly(ethylene glycol diacrylate) (PEGDA) (Mn=258 Da) at the monomer feed molar ratios [PAA]0/[PEGDA]0 = 99/1, 90/10 and 80/20. The resultant poly(PAA-PEGDA) copolymers were characterized by Proton Nuclear Magnetic Resonance (1H NMR) and Gel Permeation Chromatography (GPC) to obtain the molecular weight, copolymer composition and degree of acrylate functionality. The hydrodynamic dimensions of these copolymers at pH range between 5.0 and 7.4 were studied using Dynamic Light Scattering technique (DLS). Moreover, these hyperbranched copolymers demonstrated composition- and size-dependent membrane disruptive properties by red blood cell hemolysis assay. Poly(PAA-PEGDA) with the copolymer composition [PAA]/[PEGDA]= 68/32, obtained from the copolymerization at the monomer feed molar ratio [PAA]0/[PEGDA]0 = 99/1, demonstrated significant membrane disruptive activity.

Abstract: We present the fabrication of magnetic elastomeric membranes consisting of aligned superparamagnetic microwires embedded in a polymeric matrix. The wires are formed by the magnetic assembly of colloidal iron oxide nanoparticles (NPs) dispersed in the prepolymer matrix, during the curing of the polymer and the solvent evaporation. The appropriate combination of the NPs quantity and of the viscosity of the matrix results in the formation of thin membranes of about 10mm, containing few layers of aligned wires which after the polymer curing are blocked in fixed position into the matrix. The orientation and the dimensions of the magnetic wires depend on the direction and intensity of the external MF respectively.

Abstract: Asymmetry is intrinsic to natural systems and is widely used by living organisms for efficient adaptation, mimicry and movement. Polymer bilayers are the example of synthetic asymmetric systems, which are able to generate macroscopic motion and fold by forming different 3D objects such as tubes and capsules. Similar to bimetal films, the polymer bilayer consist of two substances with different swelling properties. One polymer is non-swellable and hydrophobic. Another polymer is water-swellable hydrogel. The folding, which might occur in response to temperature or pH, is caused by swelling of the hydrogel layer. The formed tubes and capsules can be manipulated using magnetic field. Reversible folding and unfolding of the polymer films is applied for reversible capture and release of cells in response to change of temperature and other signals. This novel biomimetic approach can be used for controlled encapsulation and release of microparticles, cells and drugs as well as fabrication of 3D scaffolds for tissue engineering.

Abstract: The transportation phenomenon of ethanol in pre-deformed poly(methyl methacrylate) (PMMA) is systematically investigated. Two different phenomena simultaneously occur during this process. One is shape recovery, which is resulted from the ethanol induced softening and plasticization of PMMA. The other is swelling, which is produced by the ethanol induced molecular relaxation. Based on this study, a novel surface patterning method is proposed to fabricate PMMA microlens arrays in a simple and cost-effective manner.